This invention relates to additive fabrication apparatus and methods, generally known as rapid prototyping techniques. Such techniques have found applications in: small batch actual production of parts or assemblies; prototyping of products to test functionality, fit for assembly, marketability, and other factors; mold, die and other tool making for other manufacturing processes such as casting, extrusion, EDM, etc.; and solid imaging of 3D data in domains such as mathematics, chemistry, medicine, etc., and sculpturing and other forms of art work.
The major advantages of additive fabrication over the subtractive techniques such as metal cutting are: the ability to produce parts with unlimited geometrical complexity; the radically new possibility of designing the internal structure of parts; the possibility of unattended and automated operation; and the possibility of wasteless and environmentally sound fabrication.
Due to the current global interest in shortening the product design and manufacturing cycle which is essential in the current competitive world market and due to the recent environmental concerns, additive processes are receiving a great deal of attention. The present invention is related to a new additive fabrication apparatus and method that has several superior features as compared with the existing techniques.
Currently available additive automated fabrication techniques are briefly described below.
Selective photocuring: The original SteroLithography and its variations fall in the category of selective photocuring. In these processes selective portions of successive layers (corresponding to cross sections of the object being built) of a special type of polymer resin are solidified by exposure to light.
Selective sintering: In this process desired sections of thin successive layers of thermoplastic or metal powder are melted such that the powder particles are melted and fused together to form cross sections of the object.
Robotically guided extrusion: This process forces thermoplastic paste through an extrusion nozzle which is moved about by a robotic arm to lay down the molten material in desired locations of each successive layer of the object.
Droplet deposition on powder: In this process an adhesive liquid is ejected, usually by techniques similar to that used in the ink jet printers, to a thin powder layer to selectively join the powder particles to form solid cross sections of the object. The composite object may be later cured by heat for improved strength. This process is usually called 3D Printing.
Adhesion of cut sheets: This process separately cuts (usually by laser) the contour of each cross section of the object on sheets of a laminating material (usually butchery paper). The cut layers are successively laminated by heat to create the final object.
Other techniques under development: Three dimensional welding in which a welding head is robotically guided to progressively melt and fuse (usually nickel-based steel) to build the desired object; controlled deposition of liquid droplets of various materials (recently metal) to progressively build the object layer-by-layer; and selective curing of photopolymer by two laser beams that meet at the curing points in a vat of liquid resin (the only method that does not use the successive layering technique) are some of the rapid prototyping processes that are currently under development. Most of these methods, especially the latter one, are far from being commercially feasible due to several major difficulties in creating acceptable part surface qualities and dimensional tolerances.
All of the current additive processes, with the exception of the two-laser method, use the layering approach which builds up the object in horizontal layers each about 0.1 to 0.25 mm thick. Consequently, 40 to 100 layers for each vertical centimeter (100 to 250 layers per vertical inch) are built by these processes.
The most popular processes are currently based on polymer selective photocuring. Although photocuring machines are the most expensive type of rapid prototyping equipment and photopolymers are the most expensive materials used in rapid prototyping, due to the accuracy of selective photocuring, this process is most widely used today. Photopolymer is an organic resin that solidifies (cures) under light in a particular range of wavelength (usually in the ultraviolet range). One of the reasons for the attractiveness of photopolymers is that they can be stored as liquid for a long time and then be solidified during manufacturing. The required light for photopolymer curing is provided either by a scanning laser beam or by a flood lamp which shines light through a masked sheet which lets the light through where the layer is to solidify.
The current photocuring systems build the object in a vat of liquid polymer. As each layer is cured, a new resin layer covers the cured layer by either vertically moving the object to a lower depth in the vat, or by pouring more resin into the vat. When the last layer is cured either the object is raised or the vat is drained to remove the fabricated object from the vat. Another approach for layer creation builds the object as it is suspended from an ascending platform. For each new layer the liquid resin is poured in a thin layer on a plate of glass just below the last layer of the object. The bottom surface of the glass platform is exposed to either laser or flood lamp light for curing.
When a laser beam is used for photocuring certain sections of each resin layer are scanned by the beam according to the 2D geometry of the related cross section of the object. The scanned sections solidify and the rest of the layer remain in liquid form. The laser method is potentially more energy efficient since it emits light only on specific surfaces and the monochromic nature of the laser beam results in more uniform curing for thicker layers.
In the flood lamp approach a mask sheet is created, usually using ordinary laser printers and and transparent sheets. For each layer a mask sheet must be created. Excessive mask sheet consumption and complication in accurately feeding each sheet to the desired position may be considered as serious disadvantages of this technique. Certain methods use a single glass sheet which is electrostatically charged in desired sections. Black toner particles are then attracted by the charged sections. After light exposure the sheet is discharged, the toner is collected, the liquid polymer in sections of the layer which are not exposed to light are vacuumed and the process is repeated for the new layer on the same glass sheet.
The flood lamp approach has two important advantages: first, the curing process for each layer may be much faster since the entire desired section of the layer is exposed instantly and simultaneously to light, and second, the broad spectrum of lamp light make the process less sensitive to variations in the polymer material, whereas a laser requires the polymer to be tuned to its specific frequency.
Major drawbacks of the current rapid prototyping equipment, and methods include the following:
a) The processes are slow (typically between 5 to 70 hours for relatively small objects). PA1 b) The parts fabricated by the current processes generally have poor surface quality. PA1 c) Parts created with most of the current methods have weak structures. PA1 d) There is a limited choice of materials that may be used. Some of these materials are relatively expensive (e.g. approximately $250 per pound for photopolymers). PA1 e) Current methods are limited to fabrication of part dimensions that are generally less than one meter is each dimension. PA1 f) The commercial machines using the current approaches are expensive (between $50K and $3M, with an average of $300K). Cost is especially high if laser or photo masking is used or if the processing machine has a large work envelope. Precision is also a big factor in the cost of the new methods.
It is an object of the present invention to provide a new and improved apparatus and method for additive fabrication of products, including apparatus and method which can be automated.